1. Field of the Invention
The present invention relates to a liquid crystal display device.
2. Description of the Related Art
A liquid crystal display device is a kind of flat-panel display device. Higher image quality, lower cost, and power saving are requested of liquid crystal display devices. For example, higher image quality is implemented by improving the color filter of the liquid crystal display device. The image quality is improved by implementing, e.g., sufficient color purity, high contrast, and high flatness.
To improve the image quality of the liquid crystal display device, liquid crystal alignment methods or liquid crystal driving methods such as VA (Vertical Alignment), HAN (Hybrid-Aligned Nematic), TN (Twisted Nematic), OCB (Optically Compensated Bend), and CPA (Continuous Pinwheel Alignment) have been proposed. By putting these techniques into practical use, a wide viewing angle and quick response of the liquid crystal display device are implemented.
A VA liquid crystal display device in which liquid crystal is aligned parallel to the flat surface of a substrate such as glass implements a wide viewing angle and quick response. A HAN liquid crystal display device effectively implements a wide viewing angle. However, higher accuracy is requested of the VA liquid crystal display device or HAN liquid crystal display device in terms of the flatness (uniformity of the film thickness and reduction of unevenness of the color filter surface) for the color filter, and electrical characteristics such as the dielectric constant.
In a high-image-quality liquid crystal display device, a technique of decreasing the thickness of a liquid crystal layer (thickness of a liquid crystal cell) is important for a quick response.
For the VA method, various improved modes have been developed, including MVA (Multi-domain Vertically Alignment), PVA (Patterned Vertically Alignment), VAECB (Vertically Alignment Electrically Controlled Birefringence), VAHAN (Vertical Alignment Hybrid-Aligned Nematic), and VATN (Vertical Alignment Twisted Nematic).
A quicker liquid crystal response, wider viewing angle, and higher transmittance are required in a longitudinal electric field type liquid crystal display device in which a driving voltage is applied in the thickness direction of a liquid crystal layer, as in the VA method.
It is difficult to set the direction in which liquid crystal molecules lie, which is initially perpendicular to the substrate surface, upon applying a voltage]. In the MVA method, a plurality of slit-like projecting portions are arranged to prevent vertically aligned liquid crystal becoming unstable upon applying a liquid crystal driving voltage. In the MVA method, a wide viewing angle is ensured by forming a plurality of liquid crystal domains in different alignment directions between a plurality of slits.
Patent literature 1 (Japanese Patent No. 3957430) has disclosed a technique of forming a liquid crystal domain by using first and second alignment regulating structures (slits).
Patent literature 2 (Jpn. Pat. Appln. KOKAI Publication No. 2008-181139) has disclosed a technique of forming four liquid crystal domains by using photoalignment. In patent literature 2, alignment processing is required a plurality of (four) times to add a strict pretilt angle (89° with respect to the horizontal direction) to each liquid crystal domain, in order to ensure a wide viewing angle. Further, in patent literature 2, a plurality of alignment axes different from each other by 90° are necessary.
Patent literature 3 (Jpn. Pat. Appln. KOKAI Publication No. 2011-248132) has disclosed the electrode arrangement of an array substrate in which the second electrode protrudes from the first electrode.
As described above, in the vertically aligned MVA liquid crystal display device, a liquid crystal domain is formed using a plurality of slits formed on a color filter in order to ensure a wide viewing angle. The slit is formed on the liquid crystal layer side, compared to the color filter. Liquid crystal molecules positioned between two resin slits have a lengthwise direction perpendicular to the substrate surface before a driving voltage is applied. When the driving voltage is applied, the liquid crystal molecules between the two slits lie in a direction perpendicular to the two slits, and tilt to be parallel to the substrate surface. However, as for a central liquid crystal molecule between the two slits, the direction in which the central liquid crystal molecule lies upon applying a driving voltage is not uniquely determined, and splay alignment or bend alignment sometimes occurs. Such liquid crystal alignment disorder causes roughness of the liquid crystal display, display unevenness, and a decrease in transmittance.
In the MVA method, it is difficult to finely control, by the driving voltage, the amount in which the liquid crystal molecules lie. For this reason, the MVA liquid crystal display device is inferior in halftone display controllability to, e.g., a horizontally aligned liquid crystal display device called an IPS. Particularly, in the MVA method, the linearity between the driving voltage and the display (response speed) is poor, and halftone display at a low driving voltage is difficult.
The technique in patent literature 2 solves many problems of the MVA method as described above. However, the technique in patent literature 2 requires alignment processing involving a plurality of exposures. In patent literature 2, the alignment sometimes becomes slightly nonuniform in a domain formed to ensure a wide viewing angle, as shown in FIG. 61 of patent literature 2. Owing to this nonuniformity, unevenness readily occurs at the peripheral portion of a pixel.
FIG. 20 is a plan view showing an example of a conventional pixel electrode which improves the nonuniformity of the tilt in a domain. Source signal lines 7a and 7b are arranged at frame portions of a pixel (two lateral sides of a pixel) in the longitudinal direction. Gate lines 23a and 23b are arranged at frame portions of the pixel in the lateral direction.
FIG. 21 is a partial sectional view showing an example of the state of an electric line of force between the distal end of the pixel electrode and the source signal lines 7a and 7b. FIG. 21 shows a section taken along a line E-E′ in FIG. 20. FIGS. 20 and 21 show the generation state of a dark portion 24 formed at the frame portion of the pixel.
In FIG. 20, a pixel electrode 25 having a comb tooth pattern in which the lengthwise directions of comb teeth extend in four directions is arranged to prevent the nonuniformity of alignment in the domain.
By the comb tooth-like pixel electrode 25, liquid crystal alignments in four domains can be changed to coincide with the lengthwise directions of the four comb teeth upon applying a liquid crystal driving voltage. Hence, a high liquid crystal image quality can be provided.
However, the distal end portion of the comb tooth-like pixel electrode 25 shown in FIG. 20 is readily influenced by the voltages of the source signal lines 7a and 7b arranged at frame portions of the pixel in the longitudinal direction. As the pixel size becomes smaller, the influence of wiring of thin film transistors (TFTs) such as the source signal lines 7a and 7b becomes more serious. Due to an electric field between the comb tooth-like pixel electrode 25 and the source signal lines 7a and 7b, a liquid crystal molecule 26 near a frame portion of the pixel in the longitudinal direction lies in a direction 29 different from the alignment in the lengthwise direction of the pixel electrode 25, and the dark portion 24 may be generated. The dark portion 24 may be ignored in a large liquid crystal display device with a large pixel size. However, in micropixels at 200 ppi (pixels per inch) or more, the area ratio of the dark portion 24 in the longitudinal direction to the pixel rises and may exert a non-negligible influence.
When the liquid crystal driving voltage is applied, liquid crystal molecules in the pixel lie in four directions 27a to 27d toward the center of the pixel which are lengthwise directions of the comb tooth-like pixel electrode 25, thereby implementing a wide-viewing-angle display. However, the liquid crystal molecules are influenced by the electric fields of the source signal lines 7a and 7b at the frame portions of the pixel. An electric field expressed by an electric line 28 of force is formed between the comb tooth-like pixel electrode 25 and each of the source signal lines 7a and 7b. By the electric line 28 of force, the liquid crystal molecule 26 near the frame portion lies in the direction 29 from the pixel electrode toward the source signal line 7a or 7b. The direction 29 in which the liquid crystal molecule 26 lies is different from a direction in which other main liquid crystal molecules lie. As a result, the light transmittance of the pixel near the frame portion becomes low, and the dark portions 24 are generated near the source signal lines 7a and 7b. 
Patent literature 3 has disclosed a vertically aligned liquid crystal display device which executes symmetric liquid crystal operations to divide the pixel region into two when the driving voltage is applied between the first, second, and third electrodes, as disclosed in claim 9 of patent literature 3. An arrangement in which color filters of a plurality of colors are stacked on a transparent conductive film disclosed in claim 1 of patent literature 3 is advantageous in ensuring a high transmittance. However, when a color filter serving as a dielectric is stacked on the transparent conductive film, it sometimes has an extra electric capacitance. In a mobile device such as a tablet terminal or mobile phone, it is desirable to reduce power consumption and use the mobile device for a long time. It is therefore preferable for the liquid crystal display device not to have an extra electric capacitance. Further, patent literature 3 has not disclosed the surface of the first electrode for performing a more uniform liquid crystal operation at a higher speed. In addition, patent literature 3 has not disclosed the pretilt direction of an alignment film on the first electrode for performing a higher-speed liquid crystal operation. Patent literature 3 has not disclosed that a pretilt angle for a high-speed operation is given to an alignment film on the first electrode excluding the shoulder portion and recessed portion of the color filter.